Diagnostic ultrasound is an essential modality in virtually every medical specialty and particularly in obstetrics, cardiology and radiology. The ultrasound transducer is the critical component and the limiting factor affecting the quality of diagnostic ultrasound imaging and Doppler measurements. The most sophisticated medical ultrasound scanners now use (N.times.1) linear arrays containing over a hundred transducer elements which may be multiplexed and/or electronically steered and focused via phased array techniques.
Two dimensional (N.times.M) transducer arrays will be essential in future diagnostic ultrasound equipment to improve clinical image quality and Doppler measurements. The most immediate clinical application of 2-D phased arrays is to reduce image slice thickness by focusing in the elevation plane perpendicular to the scanning dimension. The second important application of 2-D transducer arrays is the correction of phase aberrations introduced across the transducer aperture by tissue inhomogeneities. These aberrations occur in two-dimensions so that 2-D arrays combined with the proper phase correction signal processing are essential to restore diagnostic image quality.
In addition to improving conventional ultrasound B-scan image quality, two-dimensional transducer arrays are necessary to develop the new modes of ultrasound imaging anticipated in the near future. These new techniques include (1) presentation of simultaneous orthogonal B-mode scans; (2) acquisition of several B-scans electronically steered in the elevation direction; (3) development of high-speed C-scans; (4) high-speed volumetric ultrasound scanning to enable real time three-dimensional imaging and volumetric, angle-independent flow imaging. These techniques can only be implemented with 2-D array transducers.
It has already been a significant challenge for the ultrasound community to design and fabricate linear phased arrays for medical ultrasound over the past two decades. Three criteria have determined the size and geometry of the transducer array elements. (1) The elements must have sufficient angular sensitivity to steer the phased array over a .+-./-45.degree. sector angle. (2) The arrays must suppress grating lobe artifacts by fine inter-element spacing and (3) the width of each rectangular element must be small compared to the transducer thickness to remove parasitic lateral mode vibrations from the desired transducer pass band. These criteria have resulted in long narrow elements in linear arrays such that each element is less than one wavelength wide in tissue (e.g., 0.3mm wide.times.10 mm long at 3.5 MHz). Unfortunately, the design and fabrication problems of one-dimensional transducer arrays become almost, overwhelming when extended in two dimensions. In this case element sizes may be less than 0.2mm.times.0.2mm for more than 1000 elements in the array.
There are two obstacles which limit such transducer arrays.
(1) There are severe fabrication difficulties in electrical connection to such array elements which can be less than one ultrasound wavelength on a side.
(2) It is very difficult to achieve adequate sensitivity and bandwidth from such small elements
In the last 15 years there have been several descriptions of prototype 2-D array transducers for medical ultrasonic imaging. Some of these prototypes used integrated circuit (IC) fabrication techniques to include a large number of transducer elements, but the resulting product was unsuitable from an acoustic viewpoint. All these prototype arrays were unsuitable for modern medical ultrasound imaging in which the transducer is placed in direct contact with the patient's skin. Erickson et al. (Acoustical Holography, Vol. 7, pp 423-425, 1976), describes 8.times.8 element 2-D array (element size 2mm.times.2mm) of L.sub.i NbO.sub.S operating at 3MHz on sapphire and silicon substrates with associated integrated circuits with a 0.001 inch high bonding pad for each element. The L.sub.i NbO.sub.S /sapphire/silicone structure causes significant problems in acoustic performance. The array was designed only for water tank imaging. Plummer et al. (IEEE Trans. on Sonics and UItrasonics, 50-55, pp. 273-280, 1978), describes the fabrication of 16.times.16 element 2-D arrays operating at 2-4MHz (element size=2.2mm.times.2.2mm) of PZT connected by a conductive epoxy bump of unspecified height to a glass substrate with plated through holes on silicon integrated circuits. Again, the PZT/glass/silicon structure will cause significant problems in acoustic performance. Pappalardo (Ultrasonics, pp. 81-86, 1981), described a 23.times.23 element 2-D array operating at 1.6MHz (element size=0.8mm.times.0.8mm) in which each column of array elements is glued to the edge of a fiber-glass circuit board using conductive epoxy.
Each of these descriptions share the design of a piezoelectric element mounted on a substrate of high acoustic impedance but low acoustic losses. Thus much of the emitted ultrasound energy is emitted from the rear surface of the piezoelectric element and reverberates inside the substrate before transmission into the load (water or tissue). This would result in long pulses of narrow bandwidth, unsuited for high quality medical ultrasound. In each design no attention is paid to the bonding layer thickness whether solder or conductive epoxy.
Another group of 2-D arrays include transducers mounted on lossy acoustic backings to obtain good pulse characteristics but without the advantage of microelectronics fabrication techniques. De Franould et al. (IEEE UItrasonics Symposium, 77CH1264, pp. 251-263, 1977), reported a 2-D array transducer using PZT on a non-conducting plexiglass .lambda./4 mismatching layer and a lossy tungsten-epoxy backing (0.4mm.times.4mm elements at 2.4MHz). No connection technique was specified. Fitting et al. (IEEE Trans. on UItrasonic Farro. and Freq. Control, UFFC-34, pp. 346-356, 1987), described a 2-D array transducer for receive mode only using mask metallization of polyvinylidene fluoride on a tungsten epoxy substrate. However, none of these involved the use of multi-layer ceramic technology. Nakashani et al. (U.S. Pat. No. 4,296,349) discussed a conductive mismatching layer of thicknesses of .lambda./32 to 3.lambda./16, however, this work involved low acoustic impedance transducer material (polymer) rather than the high acoustic impedance of PZT.